Summary

When Sherlock leaves the envelope for John and Mary at their wedding, he forgets to take out another piece of paper from it - the paper on which he poured his heart out while preparing the Best Man's Speech at 221B Baker Street.

He intended to burn the piece right after he finished writing, but he was too exhausted from the emotions and accidentally slipped it amongst his composition for the Waltz.

As things work out, somebody reads his letter. That somebody is John Watson.